This invention relates to the sensing of distances in air, by means of ultrasound (or possibly audible sound) which is transmitted as a pulse from a distance sensing device and reflected back to it by an object such as a wall, serving as a target at a distance from the device. The time from transmission of the pulse to receipt of its echo is measured, and provides an indication of the distance. Equipment for this purpose is already known, for example, the Sonarange device made by Freedom Engineering Limited, the apparatus described in U.K. patent specification No. 2 082 325A of Sonic Tape PLC, and that described in German OLS No. 2 755 556A of Dieter Haffer.
In general the device will be used to measure distances. One particular application of such devices is for building surveyors and estate agents, measuring the rooms within a building.
At least some known apparatus is constructed so that it largely does not respond to echoes from off the centre line of the transmitted pulse. Nevertheless circumstances arise where a user cannot be sure which of possible target surfaces is giving the echo to which the apparatus responds. For example it may be difficult to be sure that the apparatus is measuring the distance to a wall, rather than to a table standing in front of the wall.